This invention relates to oxidation resistant coatings. More particularly, this invention relates to articles having a coating that is resistant to failure in high temperature oxidative environments and methods of protecting an article in a high temperature, oxidative environment.
Nickel (Ni), cobalt (Co), and iron (Fe) based alloys are frequently used to form articles designed for use in high temperature, highly oxidative environments. Such articles include components that are used in turbine systems, such as, but not limited to, aircraft turbines, land-based turbines, marine-based turbines, and the like. To survive in such environments, articles made of these alloys often require coatings to protect the underlying alloys against oxidation and/or hot corrosion. The coating, which is frequently a nickel aluminide (NiAl)-based material, can also be used to provide adherence to an outer thermal barrier coating, where such a coating is employed.
Performance of the nickel aluminide-based coating is adversely affected by surface-connected coating defects, cracks, and oxygen-permeable second-phase stringers. It is therefore highly desirable to reduce such defects. Grit blasting and shot-peening have been applied with marginal success to heal defects; but such techniques introduce the risk of additional coating damage.
In addition to surface defects, the physical vapor deposition processes (also referred hereinafter as xe2x80x9cPVDxe2x80x9d) that are used to deposit nickel aluminide-based coatings frequently result in the loss of aluminum during deposition. Attempts have been made to compensate for aluminum loss by either adjusting the composition of the PVD source material or using post-PVD vapor-phase aluminizing. The success of such corrective attempts has been limited by problems with castability and ductility of the PVD source, coating adhesion, coating contamination, and chemistry control.
The present approaches have not been successful in either healing surface defects or compensating for lost aluminum in such nickel aluminide-based coatings. Consequently, nickel aluminide-based coatings often do not provide adequate protection for the underlying alloy substrates in such harsh environments. Therefore, there remains a need for a coating that provides protection for a metallic article in a high temperature, highly oxidative environment. There is also a need for an article having such a protective coating. Finally, there is a further need for a method of providing an article with such a protective coating.
Embodiments of the present invention address these and other needs. One embodiment is an article comprising a metallic substrate; and a substantially single-phase coating disposed on the substrate, wherein the coating comprises nickel (Ni) and at least about 30 atomic percent aluminum (Al), wherein the coating further comprises a gradient in Al composition, the gradient extending from a first Al concentration level at an outer surface of the coating to a second Al concentration level at an interface between the substantially single-phase coating and the substrate; wherein the first Al concentration level is greater than the second Al concentration level and the second concentration level is at least about 30 atomic percent Al.
A second embodiment is a coating for protecting an article, the coating comprising a substantially single-phase coating disposed on a substrate, wherein the coating comprises nickel (Ni) and at least about 30 atomic percent aluminum (Al), wherein the coating further comprises a gradient in Al composition, the gradient extending from a first Al concentration level at an outer surface of the coating to a second Al concentration level at an interface between the substantially single-phase coating and the substrate; wherein the first Al concentration level is greater than the second Al concentration level and the second concentration level is at least about 30 atomic percent Al.
A third embodiment is a method for protecting an article from a high-temperature oxidative environment, the method comprising providing a substrate; disposing a first coating layer onto the substrate, wherein the first coating layer comprises nickel (Ni) and aluminum (Al); and disposing a second coating layer onto the first coating layer, wherein the second coating layer comprises at least about 90 atomic percent aluminum.